Avoid unnecessary single-child Append nodes.

Before commit d3cc37f1d8, an inheritance parent
whose only children were temp tables of other sessions would end up
as a simple scan of the parent; but with that commit, we end up with
an Append node, per a report from Ashutosh Bapat.  Tweak the logic
so that we go back to the old way, and update the function header
comment for partitioning while we're at it.

Ashutosh Bapat, reviewed by Amit Langote and adjusted by me.

Discussion: http://postgr.es/m/CAFjFpReWJr1yTkHU=OqiMBmcYCMoSW3VPR39RBuQ_ovwDFBT5Q@mail.gmail.com

src/backend/optimizer/prep/prepunion.c

@@ -1360,8 +1360,12 @@ expand_inherited_tables(PlannerInfo *root)
  * table, but with inh = false, to represent the parent table in its role
  * as a simple member of the inheritance set.
  *
- * A childless table is never considered to be an inheritance set; therefore
+ * A childless table is never considered to be an inheritance set. For
- * a parent RTE must always have at least two associated AppendRelInfos.
+ * regular inheritance, a parent RTE must always have at least two associated
+ * AppendRelInfos: one corresponding to the parent table as a simple member of
+ * inheritance set and one or more corresponding to the actual children.
+ * Since a partitioned table is not scanned, it might have only one associated
+ * AppendRelInfo.
  */
 static void
 expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
@@ -1374,7 +1378,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 	List	   *inhOIDs;
 	List	   *appinfos;
 	ListCell   *l;
-	bool		need_append;
+	bool		has_child;
 	PartitionedChildRelInfo *pcinfo;
 	List	   *partitioned_child_rels = NIL;
 
@@ -1448,7 +1452,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 
 	/* Scan the inheritance set and expand it */
 	appinfos = NIL;
-	need_append = false;
+	has_child = false;
 	foreach(l, inhOIDs)
 	{
 		Oid			childOID = lfirst_oid(l);
@@ -1502,7 +1506,10 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 		 */
 		if (childrte->relkind != RELKIND_PARTITIONED_TABLE)
 		{
-			need_append = true;
+			/* Remember if we saw a real child. */
+			if (childOID != parentOID)
+				has_child = true;
+
 			appinfo = makeNode(AppendRelInfo);
 			appinfo->parent_relid = rti;
 			appinfo->child_relid = childRTindex;
@@ -1582,7 +1589,7 @@ expand_inherited_rtentry(PlannerInfo *root, RangeTblEntry *rte, Index rti)
 	 * the parent table is harmless, so we don't bother to get rid of it;
 	 * ditto for the useless PlanRowMark node.
 	 */
-	if (!need_append)
+	if (!has_child)
 	{
 		/* Clear flag before returning */
 		rte->inh = false;